Allergic contact
dermatitis (ACD) referred to as contact hypersensitivity (CHS) is one the most
frequent inflammatory skin diseases characterized by redness, papules, and vesicles,
followed by scaling and dry skin. ACD is elicited upon skin contact with non-protein
chemicals called haptens and corresponds to a cutaneous delayed-type hypersensitivity
reaction, mediated by hapten-specific T cells. During the sensitization phase,
both CD4+ and CD8+ T cell precursors are activated in the draining lymph nodes
by presentation of haptenated peptides by skin dendritic cells (DC). Subsequent
hapten painting on a remote skin site induces the recruitment and activation
of specific T cells at the site of challenge leading to apoptosis of
keratinocytes, recruitment of inflammatory cells and development of clinical
symptoms. Experimental studies from the last 10 years have demonstrated that,
in normal CHS responses to strong haptens, CD8+ type 1 T cells are effector
cells of CHS through cytotoxicity and IFNg, production
while CD4+ T cells are endowed with down-regulatory functions. The latter may
correspond to the recently described CD4+ CD25+ regulatory T cell population.
However, in some instances, especially those where there is a deficient CD8
T cell pool, CD4+ T cells can be effector cells of CHS. Ongoing studies will
have to confirm that the pathophysiology of human ACD is similar to the mouse
CHS and that the CHS response to common weak haptens, most frequently involved
in human ACD, is similar to that reported for strong haptens.

Keywords:
Apoptosis; Dermatitis, allergic contact; Inflammation; Skin

INTRODUCTION

Contact Dermatitis
is a frequent inflammatory skin disease in industrialized countries with a great
socioeconomic impact and is one of the most common occupational diseases.1-3
As the outermost barrier of the human body, the skin is the first to encounter
chemical and physical factors from the environment. According to the pathophysiological
mechanisms involved, two main types of contact dermatitis may be distinguished.
Irritant contact dermatitis is due to the proinflammatory and toxic effects
of xenobiotics able to activate the skin innate immunity. Allergic contact
dermatitis requires the activation of antigen-specific acquired immunity
leading to the development of effector T cells, which mediate the skin inflammation.
It is characterized by redness, papules, and vesicles, followed by scaling and
dry skin1-3

Allergic contact
dermatitis (ACD), also referred to as contact hypersensitivity (CHS), is a T
cell-mediated skin inflammatory reaction due to repeated contact of the skin
with non-protein chemicals, called haptens.1-4 In contrast to classical
DTH, which requires intradermal injection of exogenous protein, initiation of
CHS is generated by topical application on the epidermis of sensitizing haptens
(i.e. nickel, chrome, DNFB, TNCB, oxazolone). Studies from the last 10 years
have emphasized that CD8+ T cells are the main effector cells of CHS while CD4+
T cells behave as down-regulatory cells.5-7 It is noteworthy that
there is still some controversy as to whether CD8+ T cells are effector cells
of CHS in all strains of mice and for all types of haptens.8 Similarly,
the relative contribution of CD4 and CD8 T cells in human ACD is still unclear.9
CD8+ T cells are now known to mediate DTH responses in allergic contact dermatitis,
drug eruptions, asthma, and autoimmune diseases.10 The difference
between CD8+ T cells and CD4+ T cells mediating DTH may relate to the molecular
mechanisms by which antigens are processed and presented to the T cells. Exogenous
antigens are phagocytosed and processed on MHC class II molecules (e.g., HLA-DR)
for presentation to CD4+ T cells. In contrast, cytoplasmic antigens are processed
by the endogenous pathway on MHC class I molecules (e.g., HLA-A, -B, and -C)
for presentation to CD8+ T cells. External allergens can also enter the endogenous
pathway to be presented to CD8+ T cells. These include many contact sensitizers,
chemical and protein respiratory allergens, viral antigens, metabolic products
of drugs, and autoantigens. Haptens are also able to directly interact with
peptides which are already in the groove of MHC class II and class I molecules.4
Thus CD8+ and CD4+ T cells could be activated in the lymph nodes by antigen
presenting cells (APC) expressing haptenated peptides presented in the groove
of MHC class I and class II molecules, respectively.

1. PATHOPHYSIOLOGY
OF CONTACT HYPERSENSITIVITY - General scheme

Knowledge of the
pathophysiology of ACD is derived chiefly from animal models in which the skin
inflammation induced by hapten painting onto the skin is referred to as contact
hypersensitivity (CHS). Two temporally and spatially dissociated phases are
usually necessary to achieve optimal CHS reaction: the sensitization and the
elicitation phases (Figure 1). We describe
here the well-accepted pathophysiological pathways of CHS and ACD. The sensitization
phase (also known as afferent phase) occurs at the first skin contact with a
hapten and leads to the priming and expansion of hapten specific T cells in
lymph nodes. Topically applied hapten is uptaken by skin DC, especially Langerhans
cells (LC), which migrate from the epidermis to the paracortical area of draining
lymph nodes, where they present haptenated peptide/MHC molecule complexes to
hapten-specific T cell precursors. Specific T cells emigrate from the lymph
nodes and enter the blood through the thoractic duct and recirculate in the
blood and secondary Knowledge of the pathophysiology of ACD is derived chiefly
from animal models in which the skin inflammation induced by hapten painting
onto the skin is referred to as contact hypersensitivity (CHS). Two temporally
and spatially dissociated phases are usually necessary to achieve optimal CHS
reaction: the sensitization and the elicitation phases (Figure
1). We describe here the well-accepted pathophysiological pathways of CHS
and ACD. The sensitization phase (also known as afferent phase) occurs at the
first skin contact with a hapten and leads to the priming and expansion of hapten
specific T cells in lymph nodes. Topically applied hapten is uptaken by skin
DC, especially Langerhans cells (LC), which migrate from the epidermis to the
paracortical area of draining lymph nodes, where they present haptenated peptide/MHC
molecule complexes to hapten-specific T cell precursors. Specific T cells emigrate
from the lymph nodes and enter the blood through the thoractic duct and recirculate
in the blood and secondary lymphoid organs. The elicitation phase occurs a few
hours after a subsequent skin contact with the same hapten which induces chemokine
production, endothelial cells and mast cells activation, and neutrophils infiltration,
all necessary for recruiting specific T cells. T cells interact with hapten-bearing
skin antigen-presenting cells. Activated CD8+ cytotoxic T cells produce type
1 cytokines (IFNg) and induce skin cell activation
and keratinocyte apoptosis leading to the amplification of the cutaneous inflammation
through production of a whole set of cytokines and chemokines. These latter
allowing recruitment of polymorphous cellular infiltrate characteristic of CHS.
This efferent phase of CHS lasts 72 hours in humans and 24 to 48 hours in mice.
The inflammatory reaction persists over several days and progressively decreases
upon physiological down-regulating mechanisms.

2. CHS TO STRONG
EXPERIMENTAL HAPTENS

The vast majority
of data available on CHS have been obtained with strong sensitizers, such as
DNFB, DNCB, TNP, oxazolone, which have unique chemical and immunological properties:
i) they represent a minority of the chemicals among the thousands that are able
to induce ACD in humans; ii) they are endowed with potent proinflammatory properties,
known as irritancy, due to the toxicity of the chemical. This toxicity provides
a danger signal for the skin innate immune system, leading to production of
inflammatory cytokines (IL-1, TNF) and chemokines by skin cells and to activation
of skin DC that can initiate their maturation process and emigrate to draining
lymph nodes. DC maturation and migration into lymph nodes is mandatory for initiation
of CHS, and several cytokines, chemokines and their receptors, among which CCL21
and its ligand CCR7, are important in this process.11

It is noteworthy
that strong haptens can induce a primary ACD, i.e., a hapten-specific immune
reaction, following a single skin contact, which has the same pathophysiology
as the classical CHS reaction obtained with two hapten skin paintings.12
The occurrence of CHS following a single exposure to haptens could be explained
by the persistence of haptens in the skin for several days after painting, allowing
the recruitment of specific T cells at the site of skin sensitization. Thus,
strong haptens, through their toxicity, deliver danger signals able to potently
activate innate immunity that allows the development of a robust and rapid hapten-specific
immunity.

Alternatively,
the most frequently encountered haptens, classified as moderate, weak or very
weak, are much less irritant than strong haptens and may not have the same ability
to activate innate immune cells (see chapter 3).

The respective
contribution of CD4+ and CD8+ T cells in CHS has been examined using different
strategies: i) in vivo depletion of normal mice with anti-CD4 and anti-CD8
mAbs; ii) transfer of CD4+ or CD8+ T cells from sensitized mice into immuno
incompetent Rag°/° mice; iii) use of MHC class I°/° (CD8+ T cell-deficient)
or MHC class II°/° (CD4+ T cell-deficient) mice; iv) transfer of haptenated
DC from MHC class I°/° or MHC class II°/° mice to induce CHS in naïve recipients.
Mice genetically deficient in CD4 or CD8 molecules (CD4°/° or CD8°/°) do not
represent relevant models of CD4+ or CD8+ T cell deficiency and results obtained
with these mice will be discussed later (see chapter 4 "CD4-deficient mice").

Adoptive transfer
experiments first highlighted that DTH to protein was transferable into MHC
class II-matched recipients, whereas transfer of CHS required class I-matched
recipients.13 Using in vivo depletion of CD4+ and CD8+ T cell
subsets, Gocinski and Tigelaar were the first to suggest that CD8+ T cells could
mediate the CHS response to DNFB and other strong haptens.14 They
further showed that CD4+ T cells were endowed with a down-regulatory activity,
since the CHS reaction was enhanced following in vivo depletion of CD4+
T cells.

Bour et al. used
another approach to study the contribution of CD4+ and CD8+ T cell subsets.
They studied CHS in MHC class I and MHC class II KO mice, which are deficient
in CD8+ and CD4+ T cells, respectively.15-17 Indeed, CD4+ T cells
develop during the ontogeny by interaction with thymic APCs expressing MHC class
II molecules. Thus, in the absence of MHC class II molecules, thymic precursors
cannot differentiate into CD4+ T cell and therefore most of the circulating
mature T cells are CD8+ T cells. Likewise, due to lack of positive selection
of CD8+ T cells in MHC class I-deficient mice, CD4+ T cells constitute the vast
majority of mature peripheral T cells. Surprisingly, class I°/° mice did not
develop any CHS response to DNFB, indicating that CD8+ T cells were mandatory
for the development of the disease. Since these I°/° mice have normal numbers
and functions of CD4+ T cells and can mount a classical DTH reaction to alloantigens
and to protein antigens,12-14 these data demonstrated that CD4+ T
cells do not mediate the CHS reaction to DNFB. On the other hand, class II°/°
mice developed an enhanced CHS reaction, with chronic skin inflammation, and
in vivo depletion of CD8+ T cells in these MHC mice resulted in a complete
abrogation of the CHS. More importantly, hapten-specific CD8+ effectors could
develop in the absence of CD4 T cell help. Indeed, there is no need of CD4+
T cells for the priming of CD8+ T cells in II°/° mice and the presence of CD4+
T cells has a negative effect on the intensity of the CD8+T cell-mediated CHS
response.15-18 Thus, other important information from these studies
was the characterization of MHC class II-restricted CD4+ T cells as down-regulatory
cells of CHS. Most of the data summarized above have been obtained with DNFB
in C57BL/6 (H2b)15-19 and in BALB/c (H2d) mice.20 Similar
results have been obtained with other strong haptens such as oxazolone,20,21
DMBA22 and TNP.18,23 Thus, these findings indicated that
a functional dichotomy exists between CD8+ T cells and CD4+ T cells which behave
as effector cells and regulatory cells, respectively, in CHS to strong haptens.

One of the properties
of chemically reactive haptens is their ability to simultaneously generate immunogenic
determinants for hapten-specific CD8+ and CD4+ T cells. As discussed later (chapter
5), CD4+ T cells can be effector cells of CHS to TNP in the event of CD8+ T cell
deficiency. The reason why effector cells of CHS are only confined in the CD8+
T cell pool in normal mice has recently been explained by Martin S et al.'s studies
which showed that CD8+ T cells induce Fas-mediated apoptosis of CD4+ T cells,
therefore preventing the priming/expansion of hapten-specific CD4+ T cells during
the sensitization phase of CHS.24

2.2. Priming
of specific CD8+ and CD4+ in lymphoid organs during the sensitization phase
of CHS

CD8+ type 1 cells
and CD4+ type 2 cells

The optimal time
between hapten painting and T cell priming is 5 days in mice. At that time,
T cells recovered from lymph nodes are endowed with potent proliferative activities.19,25
Analysis of cytokine production by CD4 and CD8 T cell subsets after in vitro
restimulation by haptenated APCs has shown that CD8+ T cells produce type 1
cytokines, mostly IFNg, while CD4+T cells produce
type 2 cytokines, including IL-4, IL-5 and IL-10.21 Analysis of the
frequency of DNFB-specific CD8+ T cells by IFNg,
ELISPOT showed an average of 50 CD8+ T cell precursors/105 lymph
node cells at day 5 post sensitization, a number which is similar to that found
in other antigen-specific immune responses.26

MHC restriction
of hapten specific T cells

Investigators from
different groups provided evidence that hapten presentation to T cells in CHS
was MHC restricted and thus similar to the presentation of protein antigens
in classical DTH.18,23 Immunization of mice with hapten-pulsed DC
recovered from the epidermis or derived from bone-marrow precursors is able
to prime specific T cells that proliferate to DNFB in secondary proliferative
responses. Immunization procedures using DC recovered from MHC class I or MHC
class II-deficient mice confirmed the opposite functional effects of the CD8
and CD4 T cell pools.19 In these experiments, MHC class I-expressing
DC (either from normal mice or from MHC I+/II- mice) induced the priming of
CD8+ T cells in the lymph nodes (assessed by specific proliferation) and the
CHS reaction upon subsequent challenge. Conversely, immunization by DC lacking
MHC class I molecules (recovered from MHC class I- deficient mice) was inefficient
at inducing a CHS reaction but could prime CD4+ T cells. Indeed, CD4+ T cells
purified from the lymph nodes of such mice were hapten-specific, as assessed
in secondary proliferative responses.19 These results were confirmed
by a recent study in non-genetically modified mice, using bone-marrow-derived
DC, which were pulsed with trinitrophenyl (TNP)-derived peptides and administered
intradermally to generate a CHS reaction. Two types of peptides that have affinity
for either MHC class I or class II peptides were used. Martin et al. showed
that class I binding peptides induced CHS responses similar to that obtained
with epicutaneous TNP application. In contrast, DC pulsed with class II binding
peptides did not sensitize for optimal CHS.27 On this basis, Cavani
et al. speculated that the ability of chemical haptens to drive CD8+ T cell
activation might be associated with their capacity to directly bind to self
peptides present in the groove of MHC-class I or class II molecules.6

CD8+ T cell priming
does not require CD4+ T cell help

Classically, optimal
activation of naïve CD8+ T cells requires signals received by CD4+ T cells,
and referred to as CD4+ T cell help. During CHS to strong haptens, CD8+ T cell
activation in lymph nodes does not require CD4+ T cell help nor contribution
of CD40/CD40-L interaction, since CD40- L-deficient mice mount a normal CHS
to DNFB.28 Mice with CD4+ T cell deficiency induced by either in
vivo treatment with anti-CD4 mAb or by genetic disruption of MHC class II
genes (MHC class II KO mice) develop a strong CHS response to haptens.19
The fact that CHS can develop in the absence of class II-restricted CD4+ T cells
was further confirmed by the observation that CHS could be induced by immunization
with hapten-derivatized DC from class II°/° mice19 and with DC from
wild type mice pulsed with haptenated MHC class I peptides.27

Other studies on
viral-induced DTH responses have indicated that activation of naive CD8+ T cells
for the generation of MHC class I restricted immune responses can occur in the
absence of T cell help.29,30 Recently, it was demonstrated that the main parameter
dictating the requirement or not of CD4 help was the number of CTL precursors
which could be activated at the time of priming.31,32 CTL responses
induced by cross-priming can be converted from CD4-dependent to CD4-independent
by increasing the frequency of CTL precursors. In the absence of CD4 T cells,
high numbers of CTL precursors were able to expand and become effector CTLs.32
The ability of high frequencies of CD8 T cells to override help was not due
to their ability to signal CD40 via expression of CD154. These findings suggest
that when precursor frequencies are high, priming of CD8 T cell responses may
not require CD4 T cell help. Another explanation for the development of CD8+
effector cells is that antigens that have the intrinsic ability to induce DC
maturation bypass the need for CD4 help via CD40 activation.33 Indeed,
mice depleted in CD4+ T cells can be primed for CTL responses by transfer of
LPS-activated, antigen-pulsed DC. In CHS, DC maturation induced by haptens with
strong inflammatory capacities may bypass the need for CD4 help via CD40/CD40-L
interaction and may be sufficient to trigger specific CTL responses with a high
precursor frequency.

2.3. Elicitation
phase of CHS is due to the recruitment and activation of CD8+ cytotoxic T cells
(CTLs)

Since the main
function of CD8+ T cells is cytotoxicity, the observation that CHS was mediated
by CD8+ cells raised the possibility that cytotoxicity was mandatory for expression
of CHS. CD8+ CTLs are effector cells of the immune defense system against viruses
and tumors35 and exert their lytic functions through two main independent
mechanisms.35 The secretory pathway involves the release of perforin and granzymes
from cytolytic granules. The non secretory pathway involves interaction of Fas-L
upregulated during T cell activation, with the apoptosis-inducing Fas molecule
on the target cell.

Although mice deficient
in either perforin (p°/°) or Fas-L (gld mutant) were equally capable to develop
a normal CHS response to DNFB, and contained hapten-specific CD8+ CTLs able
to kill haptenated targets, mice double deficient in both Fas-L- and perforin
lack specific CD8+ CTLs and could not develop CHS. This demonstrated that cytotoxicity
was necessary for the development of the pathologic process and that one cytotoxic
pathway could compensate the absence of the other.26 The CTLs contribution
to the effector phase of CHS was further demonstrated by skin analysis at the
site of hapten challenge. Akiba et al. demonstrated that CD8+ T cells could
infiltrate the challenged skin as early as 9 hours after skin painting and that
the kinetics of CD8 recruitment paralleled appearance of IFNg,
transcripts and apoptosis of skin epidermal cells.20 Double staining
for MHC class II and apoptotic cells (TUNEL staining) revealed that keratinocytes
were the main target of CTLs. Thus, CD8+ T cells are endowed with in vivo
cytotoxic activity and keratinocytes behave as antigen presenting cells during
the elicitation phase of CHS. Whether other cell types contribute to in situ
activation of hapten- specific CTLs remains unknown. Indeed, haptens rapidly
diffuse through the epidermis and could be presented as haptenated peptides
by several class I-expressing cell types in dermis including DC, endothelial
cells, and mast cells which could contribute to the activation of CD8 effectors
recruited in the skin. In this respect, Biederman et al. reported that recruitment
of neutrophils via MIP-2 exclusively produced by activated mast cells was necessary
for the development of CHS.36

Recent studies
have brought new insights in the precise steps leading to recruitment of effector
T cells in the skin upon hapten challenge. P. Askenase's team has shown that
the elicitation of CHS starts by an innate inflammatory skin reaction peaking
2 hrs after challenge, called CHS-initiating phase, which is followed by the
classical DTH-like reaction peaking at 24-48 hrs, referred to as CHS-effector
phase.37 The CS initiation phase is due to hapten binding to specific
IgM antibodies produced rapidly after the sensitization by B1-B cells. The hapten-IgM
immune complexes activate complement leading to production of C5a which behaves
as a chemotactic factor for T cells.38 Other chemokines have been
implicated in the recruitment of effector cells, among which are IL-8, MCP-1,
and RANTES.39 CXCL-1 (Groa) is produced
as early as 30 minutes after challenge and is responsible for the infiltration
of the skin by neutrophils, a mandatory step for the recruitment of effector
T cells.40,41

Apoptosis is involved
in several skin pathologies and is not restricted to CHS. In ACD, several reports
have emphasized the existence of apoptotic processes involving the epidermis.5
More recently, Trautmann et al. have demonstrated that skin lesions of atopic
dermatitis (AD) were associated with the occurrence of massive apoptosis of
epidermal cells.42 They also demonstrated that, in AD, memory/effector
T cells bearing the cutaneous lymphocyte associated antigen (CLA) and CD45 RO
are undergoing activation-induced cell death, skewing the immune response toward
surviving Th2 cells.43 Although the contribution of CTLs in the pathophysiology
of AD is not precisely known, these data emphasize that epidermal cell apoptosis
is a common feature of eczematous dermatoses and suggest that anti-apoptotic
drugs could be new therapeutic tools in these diseases.44

2.4. CD4+ T
cells down-regulate the CHS reaction

Regulatory cells
are key actors in maintaining peripheral tolerance and controlling inflammatory
responses and have been reported to inhibit the development of autoimmune and
allergic immune responses in many experimental models.6,45,46 Three
main regulatory CD4+ T cell subsets have been identified: i) antigen-specific
Tr1 cell clones which produce high amounts of the immunosuppressive cytokine
IL-10; ii) antigen-specific Th2-type CD4+ T cells which antagonize type 1 T
cell effectors characteristic of CHS; iii) and naturally occurring CD4+CD25+
T cells.

CD4+ T lymphocytes
behave as down-regulatory cells and most likely regulate both the sensitization
and elicitation phases of CHS. The frequency of hapten-specific IFNg,
producing CD8+ T cells in skin draining LNs on day 5 after hapten sensitization
is much higher in CD4+ T cell deficient mice than in normal mice, suggesting
that CD4+ T cells control the development of the CD8+ T cell pool.47
It is also possible that CD4+ T cells migrating to the challenge site contribute
to the control of inflammation and its resolution.20 Indeed, in the
absence of CD4+ T cells, mice develop a more pronounced and persistent inflammation.14,15,21,48
Moreover, CD4+ T cells are recruited in the challenged skin several hours after
recruitment of CD8+ T cells.20 Thus regulatory CD4+ T cells may control
the magnitude of the CHS by regulating in situ activation and functions of CD8+
CTL effectors.

Limited information
is currently available regarding whether a particular subset of regulatory cells
is involved in the regulation of CHS. Nickel-specific Tr1 cells (producing high
amounts of IL-10) have been cloned from skin lesions of ACD patients suggesting
that this subset of regulatory cells might contribute to the regulation of the
efferent phase of contact sensitivity.9 Indirect evidence for the
implication of CD4+CD25+ cells comes from the observation that IL-2-IgG2b fusion
protein inhibited CHS and increased the size of the CD4+CD25+ T cell compartment.49
Our own data, in the model of CHS to DNFB support a role for CD4+CD25+ regulatory
T cells in the control of the skin inflammatory response and in the establishment
of oral tolerance to haptens.46 However, the relative contribution
of the regulatory CD4+ T cell subsets in the control of ACD and CHS remains
to be clarified.

The mechanisms
by which CD4+ T cells control the development of specific CD8 T cells and the
magnitude of the CHS reaction are still poorly understood. Recent data by Gorbatchev
and Fairchild suggest that CD4+ T cells may regulate the CHS responses by eliminating
the hapten- presenting APCs in lymphoid organs during the sensitization phase
in a Fas-L dependant mechanism.50 The consequence of this would be
a limited access of CD8+ T cells to MHC class I/hapten complexes and costimulatory
signals provided by DC during priming for CHS response.

3. CHS TO COMMON
WEAK HAPTENS

In contrast to
CHS against strong haptens, there is limited information on the effector cells
mediating CHS to moderate, weak, and very weak haptens. The main reason is lack
of reproducible animal models for these weak haptens. Indeed, attempts to develop
ACD to weak haptens in normal mice have so far failed. Several non mutually
exclusive hypotheses could explain why repeated skin contact with weak haptens
cannot induce ACD: i) at variance with strong haptens, weak haptens do not bear
intrinsic pro-inflammatory properties and therefore cannot deliver danger signals
mandatory for activation, differentiation, and migration of skin DC to draining
LNs; ii) skin contact with weak haptens generates specific T cells without effector
functions due to their low frequency and/or to the weak affinity of their T
cell receptors; iii) consequently, CHS to weak haptens may be more sensitive
to regulatory/suppressive T cells which could efficiently prevent priming of
specific effector T cells.

Strong haptens,
e.g. DNCB and oxazolone are toxic molecules able to deliver danger signals to
skin cells resulting in tissue inflammation within minutes/hours following skin
contact. In contrast, weak haptens are unable to induce skin inflammation at
the site of skin contact or may generate a mild skin irritancy only, even when
used at high concentrations. We postulate that the pathophysiology of ACD to
weak haptens is similar to that of ACD to strong haptens and involves CD8 effectors
able to infiltrate the skin in sensitized individuals. Ongoing studies in our
laboratory are currently testing this hypothesis.

4. CD4+ T CELLS
CAN FUNCTION AS CHS EFFECTORS IN CERTAIN SITUATIONS

Although most recent
studies have emphasized that CD8+ T cells are the main effectors mediating CHS,
it is possible that CD4+ T cells and other cell types may act as CHS effectors
in certain experimental conditions.

4.1. Particularity
of some chemicals

Some chemicals,
e.g. fluorescein isothiocyanate (FITC) and formaldehyde, have been postulated
to provoke preferential type 2 cytokine production by CD4+T cells, which have
been shown to mediate the CHS reaction.51,52

4.2. CD4-deficient
mice

Kondo et al., and
Wang et al.'s studies concluded that CD4+ T cells were effectors of CHS, showing
that CHS to DNFB and oxazolone was greatly impaired in CD4°/° mice, genetically
deficient in the CD4 molecule.53,54 The reason for the discrepancy
between these results and those reported by other investigators most likely
relies on important functional differences between CD4 deficient, CD4+ T cell
depleted mice, and MHC class II-deficient mice. Indeed, despite disruption of
the CD4 gene, CD4°/° mice contain double negative CD4- CD8- cells exerting the
normal functions of CD4+ T cells found in normal mice. In this respect, efficient
thymic maturation of helper T cells has been shown to occur in CD4°/° mice.
Indeed, CD4neg TCR ab+ cells of CD4°/° mice allow
them to control Leishmania infections, mediate antibody class switch, and DTH
reaction to KLH.55-57 Thus, although CD4°/° mice do not express the
CD4 molecule, they are able to mount MHC class II-restricted responses, suggesting
that the CD4 coreceptor is dispensable for efficient recognition of antigens
presented by MHC class II molecules on antigen presenting cells.

4.3. CD4+ T
cells may be effector cells in CHS to haptens when CD8+ T cells are deficient

CD4+ T cells could
be effector cells in CHS to some haptens when the CD8+ T cell population is
deficient. Evidence came from studies of Martin et al., who first showed that
dendritic cells (DC) pulsed with TNP-derivatized peptides that have affinity
for class II molecules could induce a low, albeit significant CHS reaction.27
Next, they used C57BL/6 mice and class I°/° mice and studied the CHS to DNP
and TNP. CHS to DNP was normal in C57BL/6 mice and absent in class I°/° mice,
as previously reported.15 TNP was able to induce a CHS response in
C57BL/6 which was inhibited by in vivo depletion of CD8+ T cells using
specific mAbs. Surprisingly, class I°/° mice were able to develop a normal CHS
reaction to TNP which was abrogated by depletion of CD4+ T cells.24

4.4. Contribution
of other cell types in CHS

Besides TCR ab+
T cells which represent hapten-specific CHS effectors, other lymphoid cell subsets
have been shown to contribute to the complex pathway ultimately leading to the
CHS response.58,59 B-1 cells are activated in lymphoid organs within
hours after skin sensitization and produce IgM antibodies. These antibodies
diffuse in the skin and will bind the hapten immediately after the challenge.
The presence of immune complexes turns on complement activation that seems mandatory
for the recruitment of effector T cells at the challenge site.60
Recent data suggest that T-cell recruitment depends on this early antigen-specific
"initiation process" of CHS which is mediated by activation of IL-4 producing
liver NKT cells. Epicutaneous immunization causes a rapid and dramatic increase
in the percentage of liver iNKT which doubles within 2 hours and remains elevated
for up to 24 hours.37

Neutrophils play
an important role in the development of CHS. In their absence CHS is reduced.
From the literature, they are involved in the CHS-initiating and the CHS-effector
phase of the disease. Neutrophils are among the first cells to be recruited
after challenge of sensitized mice through the chemotactic effect of CXCL-1
(Gro a)40 and they appear before the infiltration
of effector CD8 T cells. Once effector cells have been activated, another influx
of neutrophils is secondary to the activation of mast cells which produce TNFa
and CXCL-2 (MIP-2).36

CONCLUSIONS

In summary, ACD
can be viewed as the result of activation of two distinct T cell subsets endowed
with opposite functions: effector T cells and downregulatory T cells. The severity
and the duration of the skin inflammation appear directly related to the respective
activation state and/or size of these two compartments. Thus, overwhelming regulation
in sensitized individuals may lead to lack of inflammation (tolerance) despite
repeated exposures to the hapten, while defects in the number or functions of
regulatory cells may explain chronic contact dermatitis. Further studies will
have to address the possibility of reversing an established ACD by either targeting
the effector T cell population, preventing the recruitment of leucocytes into
the skin39 or increasing the number or functional properties of regulatory
T cells.

7. Haptenated peptides
may be expressed on dendritic cell surface as a result of:

a) direct interaction
with peptides anchored in the antigen-binding grooves of MHC molecules
b) endocytosis of extra-cellular haptenated peptides and expression in association
with MHC class II molecules
c) direct penetration of the cellular wall and expression in association with
MHC class I molecules
d) all of the above

a) involves
production of IL-1 and IL-2 by Langerhans cells
b) occurs by means of chemical toxicity that stimulates CD4+ cells
c) occurs after activation of innate immune response cells
d) can not occur following a single exposure

14. In MHC class
II KO mice:

a) the circulating
cells are CD8+ with no CHS to strong haptens
b) the circulating cells are CD4+ with no CHS to strong haptens
c) there is deficiency of CD8+ cells with CHS to strong haptens
d) there is deficiency of CD4+ with CHS to strong haptens

15. During the
sensitization phase of CHS:

a) the CD4+
cells produce IL-4, IL-5, IL-10
b) the Langerhans cells produce IL-12 and several chemokines
c) the CD8+ cells have no proliferative activity
d) all of the above answers answers

16. During the
elicitation phase of contact dermatitis

a) the Langerhans
cells produce chemokines that stimulate CD4+ cell
b) the keratinocytes behave as antigen presenting cells and may be subject
to cytotoxicity
c) cytotoxicity is mainly mediated by CD4+ cells
d) none of the above answers answers

17. Recruiting
T cells in the skin after hapten challenge:

a) occurs after
activation of complement and production of C5a
b) involves participation of IL-8, MCP-1 and RANTES
c) is preceded by neutrophyl infiltration
d) all of the above answers

18. Epidermal cell
apoptosis:

a) is described
in contact dermatitis
b) may occur in atopic dermatitis and in other eczemas
c) could be treated with anti-apoptotic drugs
d) all of the above answers